SAFETY FIRST: Protective gloves and eyewear are recommended at this point.
The brake light should come on after the rear brake pedal has traveled approximately 20 mm or 3/4 of an inch. The rear brake light switch adjustment should be checked whenever the rear brake pedal height is changed, the rear brakes have been adjusted and whenever the rear wheel is installed or the drive chain has been adjusted.
Hold the rear brake light switch and turn the knurled plastic adjuster nut. Do not allow the switch itself to turn as this could damage it. Turning the adjuster nut clockwise when viewed from above will make the brake light come on sooner, turning it counterclockwise will make it come on later. The adjuster nut is self locking.